Remove unused field Type.Siggen in go.go.

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: Ia61fe65a226c913fbf4a11a71d7453c56aa46c0e
Reviewed-on: https://go-review.googlesource.com/14372
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>